Magnetic memory with self-aligned magnetic keeper structure

ABSTRACT

A magnetic tunneling junction (MTJ) memory cell is formed with a keeper structure on its upper conductor (write line). The keeper structure is formed by a self aligned process as three pieces: two vertical soft magnetic side pieces contacting an upper soft magnetic layer. The structure so formed completely surrounds an upper conductor and terminates on a horizontal extension of the MTJ sense layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the use of the relative magnetizationorientation of sense and reference layers of magnetic tunnel junction(MTJ) devices (cells) to provide a logic storage function for suchdevices in non-volatile memory cell arrays. In particular it relates tothe design and fabrication of cells that include a keeper structureproviding a flux closure path to direct demagnetization fields away fromthe sense layer and thereby improve the thermal stability of the cell.

2. Description of the Related Art

The magnetic tunnel junction (MTJ) basically comprises two layers offerromagnetic material, a sense layer and a reference layer, separatedby a tunnel barrier layer, which is a thin layer of insulating material.The tunnel barrier layer must be sufficiently thin so that there is aprobability for charge carriers (typically electrons) to cross the layerby means of quantum mechanical tunneling. The tunneling probability isspin dependent, depending on the orientation of the electron spinrelative to the magnetization direction of the ferromagnetic layers.Thus, if these magnetization directions are varied, the tunnelingcurrent will also vary as a function of the relative directions for agiven applied voltage. As a result of the behavior of an MTJ, sensingthe change of tunneling current for a fixed potential can enable adetermination of the relative magnetization directions of the twoferromagnetic layers that comprise it. Equivalently, the resistance ofthe MTJ can be measured, since different relative magnetizationdirections will produce different resistances.

The use of an MTJ as an information storage device requires that themagnetization of at least one of its ferromagnetic layers, the senselayer, can be varied relative to the other, the reference layer, andalso that changes in the relative directions can be sensed by means ofvariations in the tunneling current or, equivalently, the junctionresistance. In its simplest form as a two state memory storage device,the MTJ need only be capable of having its magnetizations put intoparallel or antiparallel configurations (writing) and that these twoconfigurations can be sensed by tunneling current variations orresistance variations (reading). In practice, the ferromagnetic senselayer can be modeled as having a magnetization which is free to rotatebut which energetically prefers to align in either direction along itseasy axis (the direction of magnetic crystalline anisotropy). Themagnetization of the reference layer may be thought of as beingpermanently aligned in its easy axis direction. When the sense layer isanti-aligned with the reference layer, the junction will have itsmaximum resistance, when they are aligned, the minimum resistance ispresent. In typical MRAM circuitry, the MTJ devices are located at theintersection of current carrying lines called word lines and bit lines(or word lines and sense lines). When both lines are activated, thedevice is written upon, ie, its magnetization direction is changed. Whenonly one line is activated, the resistance of the device can be sensed,so the device is effectively read.

As such cells become increasingly small in cross-sectional area andvolume, the stability of the magnetization direction of the sense layeris easily affected by random external fields, its self-demagnetizingfields and by thermal effects. One approach to increasing cell stabilityis to provide the cell with a “keeper” structure, whose role is toprovide a flux closure path that directs demagnetization fields awayfrom the cell, thereby enlarging its effective volume. Anthony (U.S.Patent Application Publication No.: US2002/0055190 A1) provides a keeperstructure which is a soft magnetic material surrounding a currentcarrying conductor beneath the sense layer. The structure therebyprovides a mechanism for preventing the formation of demagnetizationfields in the edge region of the sense layer above it. Anthony (U.S.Pat. No. 6,358,757 B2) provides a method for forming an array of MRAMdevices at the intersections of orthogonally crossing upper and lowerconductors in which the lower conductors are surrounded by soft magnetickeeper layers. Tuttle (U.S. Pat. No. 6,413,788 B1), within a variety ofembodiments, teaches methods for forming keeper structures around bothupper and lower conductors in damascene type trench configurations. Inone pertinent embodiment, a tunneling magnetoresistance (TMR) structure(called, by the inventor, the “bit region”) is formed between twoorthogonally positioned shielded damascene conductor elements. Tuttle(U.S. Pat. No. 6,417,561 B1) also provides a magnetic memory deviceformed by the methods of the previous cited patent.

A problem arises with the fabrication of such keeper structures in thatit is difficult to maintain alignment between the keeper and the senselayer during the fabrication of the cell. Tuttle (in two patents citedabove) discloses a keeper structure which is self-aligned with an upperconductor, but the upper conductor is not self-aligned with the celljunction (MTJ). In accord with the method of Tuttle, two separatephoto-processes are required to form the device. The purpose of thepresent invention is to provide a novel design and fabricationmethodology which includes the self-aligned formation of the topconductor and its keeper with the MTJ cell. This process provides aneffective flux closure for shielding the sense layer of the cell in amanner that will permit the scaling down of cell formation to very highdensity MRAM formations.

SUMMARY OF THE INVENTION

A first object of this invention is to provide an MTJ memory devicehaving a sense and reference layer and a method of forming a keeperstructure for that device to direct demagnetization fields away from itssense layer by containing the flux of said demagnetization fields.

A second object of this invention is to provide such a device andformation method wherein the keeper structure can be formed withoutmisalignment with the sense layer.

A third object of the invention is to provide a method for forming suchdevices and keeper structures that can be extended to very smalldimensions.

These objects will be achieved by a method of forming an MTJ magneticmemory cell configuration having a top keeper structure (a keeperstructure formed on its upper conductor or sense line). The fabricationmethod includes the use of the keeper structure itself as an ion-beametch mask during fabrication to self-align the sense layer with thekeeper structure. This self-aligned method of forming keeper structurecomprises a series of process steps that includes a novel patterning ofthe sense layer to allow the self-aligned formation of the keeper layer.The precise alignments provided by this method will be advantageous inthe formation of increasingly smaller memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-d are schematic illustrations of the process steps required toform the present invention. More specifically:

FIG. 1 a is a schematic diagram showing a lower conductor (the wordline) formed on a substrate in a surrounding insulator layer and sharinga common upper planar surface with the insulating layer.

FIG. 1 b is a schematic diagram showing the lower conductor (thesurrounding insulator not being shown) on which is formed an initialconfiguration of the MTJ device (reference layer, tunneling barrier,sense layer and a first capping layer).

FIGS. 1 c and 1 d are perspective and cross-sectional schematic diagramsshowing a patterned four layer configuration comprising an upperconductor (the sense line), an antiferromagnetic layer, soft magneticlayer and a second capping layer, all formed over and transverse to theMTJ device using a photolithographic process.

FIG. 1 e is a schematic diagram showing a patterning by ion-beam etch toremove a portion of the sense layer.

FIG. 1 f is a schematic diagram showing a soft magnetic layer depositedby chemical vapor deposition (CVD) over the fabrication of FIG. 1 e.

FIG. 1 g is a schematic diagram showing the results of a second ion-beametch to remove lateral portions of the soft magnetic layer and the senselayer beneath it and complete the keeper structure.

FIG. 2 shows a preferred embodiment of a reference layer which is apinned synthetic antiferromagnetic layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the invention teaches the formation of anMTJ memory cell having a top keeper structure, ie. a keeper structureformed on an upper conductor (the sense line), the formation process is,thereby, “self-aligned,” using the keeper structure itself as a mask toalign the keeper with the conductor structure.

Referring first to FIG. 1 a, there is shown a patterned lower linearconductor layer (the word line) (10) formed with a width W₁ on asubstrate (5) through any of well known methods such as plating,sputtering or evaporation in conjunction with patterning using a photomask. The conductor material is preferentially Cu, Al, or a layeredstructure such as Ta/Au/Ta and said materials are deposited to athickness between approximately 300 and 10,000 angstroms. An insulatorlayer is deposited as a blanket layer over the conductor and an uppersurface is planarized (by, for example, chemical-mechanical polishing)to form a common planar surface containing an upper conductor surface(11) and surface portions (20) of the insulator layer laterally disposedon either side of the conductor. The present figure shows the conductorand surrounding insulator after the planarization. The following figureswill show neither the insulator, as its presence will block the displayof structure features, nor the substrate layer.

Referring next to FIG. 1 b, there is shown, schematically, theplanarized lower conductor layer of FIG. 1 a (10) upon which aferromagnetic reference layer (30), an insulating tunneling barrierlayer (40), a ferromagnetic sense layer (50) and a first capping layer(60) have been deposited sequentially and patterned to form a layeredMTJ stack structure (100) of a common width W₂(W₂<W₁) centrally alignedon (10), co-linear with and longitudinally extending along (10). W₂ranges between approximately 0.1 and 1.0 microns and W₁ ranges betweenapproximately 0.15 and 2.0 microns. The patterning is accomplishedpreferentially through photolithographic lift-off processes usingreactive-ion etching (RIE) which are known in the art. The referencelayer (30) may be a layer of ferromagnetic material such as CoFe, Co,CoFe/NiFe, NiFe, CoFeB or NiFeB formed in thicknesses betweenapproximately 30 and 200 angstroms, or it may be a synthetic exchangecoupled antiferromagnetic (SYAF) layer (illustrated schematically inFIG. 2) pinned by an antiferromagnetic (AFM) layer having a structuresuch as: seed/AFM/CoFe (or Co)/Ru (or Rh)/CoFe (orCo), wherein the seedlayer is NiFe, NiFeCr, NiCr, the AFM layer is PtMn, IrMn, FeMn or NiMnand the Ru or Rh layers (providing antiferromagnetic coupling) areformed to a thickness of approximately 7.5 or 5 angstroms respectively.The tunneling barrier layer is preferentially a layer of Al₂O₃ or HfOand is formed to a thickness between approximately 5 and 30 angstroms.The capping layer is preferentially a layer of Ta, Ru, TaO, Cu, NiFeCror NiCr and is formed to a thickness between approximately 10 and 50angstroms.

Referring next to both FIGS. 1 c and 1 d, there is shown, schematically(a perspective view in 1 c and a transverse cross-sectional view throughthe center of the MTJ stack in 1 d)), the structure of FIG. 1 b uponwhich the fabrication of an upper conductor with a keeper structure isbegun, utilizing the method of the present invention. It will beunderstood by practitioners in the art that an insulating layer (200),which is partially shown in dashed-outline in FIG. 1 c, is first formedover the entire structure of FIG. 1 b and is then planarized to createplanar upper surfaces (300) which are co-planar with an upper surface(61) of the capping layer (60) of the MTJ junction stack (100). Such aplanar surface is required to support the subsequent upper conductorformation, which will now be described.

Referring to either FIG. 1 c or 1 d, there is seen a patterned,four-layer, upper linear conductor structure (150) on which a keeperstructure has not yet been fully formed. This structure comprises anupper conductor layer (the sense line) (110), and additional layers thatwill form the keeper structure: an optional antiferromagnetic (AFM)layer (120), a soft magnetic layer (130) and a capping layer (140). Thepatterned structure is transverse to the MTJ stack (100). This patternedconductor structure is formed by sequential layer depositions usingchemical vapor deposition (CVD) or ion beam deposition (IBD), thedepositions being followed by photolithographic patterning using alift-off mask and etch process. The specific photolithographicpatterning process is not shown.

The layers ((110), (120), (130) and 140) are all formed sequentially byion-beam deposition (IBD) or chemical vapor deposition. Followingdeposition, the layers are patterned by a photolithographic lift-offprocess (not shown) to achieve a linear form, having substantiallyvertical sides separated by a width W₃ (substantially equal to W₂) asshown in the figure, and the photolithographic mask is then removed.FIG. 1 c shows the formation subsequent to mask removal. Note that theupper surface (61) of capping layer (60) is exposed by removal of themask. Also note that the structure (150) is transverse to the MTJ stackand contacts it at the upper surface (61).

As is also seen in the figure, the layers ((110), (120), (130) and(140)) are contiguous and coextensive with each other and the firstlayer in the keeper structure (the AFM layer, if used, or the softmagnetic layer if the AFM layer is omitted) is coextensive with theupper conductor layer (110). The optional AFM layer is preferentiallyformed of IrMn to a thickness between approximately 30 and 100 angstromsand it may be used to provide an exchange field along the longitudinaldirection of the patterned layered structure. The soft magnetic layer ispreferentially a layer of CoFe, NiFe or NiFeCo formed to a thicknessbetween approximately 50 and 500 angstroms. The second capping layer(140) is preferentially a layer of Ta, Ru, TaO, Cu, NiFeCr, or Ni Cr andit is formed to a thickness between approximately 10 and 1000 angstroms.The as-deposited structure ((100), (120), (130) and (140)) is shown bothin perspective and in a cross-sectional view taken through alongitudinal plane bisecting the lower conductor and MTJ structure(FIGS. 1 c and 1 d (respectively)).

Referring next to FIG. 1 e, there is shown a schematic cross-sectionalview of the fabrication as shown in FIG. 1 c or 1 d, subsequent to afirst ion-beam etching. The etching process (arrows), which is donesubsequent to the removal of the photoresist mask used in the depositionprocesses described in FIG. 1 c, removes two side portions (145) of thefirst capping layer (60) (shown as shaded regions with broken-lineboundaries) which are not covered by the upper conductor layer (110) andare laterally disposed to (110). The same etch also partially removes anupper portion (146) of the sense layer (50) of the MTJ (also shownshaded and with broken-line boundaries) which is beneath (145).

Referring next to FIG. 1 f, there is shown a schematic cross-sectionalview of the fabrication in FIG. 1 d wherein a layer of soft magneticmaterial (250) such as NiFe, CoFe, NiFeCo is preferentially deposited bychemical vapor deposition (CVD) to a thickness between approximately 50and 1000 angstroms to cover all horizontal and vertical exposed surfacesof said fabrication. As is seen in the figure, layer (250) contacts theupper surface of capping layer (140) as well as the partially etchedupper surface of sense layer (50) and the lateral sides of layers (50),(60), (110), (120), (130) and (140).

Referring next to FIG. 1 g, there is shown, schematically, thefabrication of FIG. 1 e wherein a second ion-beam etch (arrows), guidedand aligned by the layer (200), has been applied vertically to removelaterally disposed portions of sense layer (50), the removed portionsbeing shown outlined in broken lines and shading (57). The ion-beam etchalso removes all portions of the second soft magnetic material layer((200) in FIG. 1 e) except for vertical side portions (201) which remainas shown contacting the sides of the sense layer (50) (partially) andterminating on horizontal projections (55) of the sense layer producedby the first ion-beam etch, the first capping layer (60), the upperconductor (110), the optional AFM layer (120) and the first softmagnetic layer (130). Advantageously, the etch is aligned by the edgesof the vertical portions of the second magnetic layer (200). The etchhas also removed the second capping layer ((140) in FIG. 1 c), but thesoft magnetic layer (130) remains. The keeper structure of the presentinvention is seen to be the soft magnetic formation comprising the sidepieces (201) formed of soft magnetic material, and the soft magneticlayer (130).

Referring finally to FIG. 2, there is seen a schematic cross-sectionalview of an isolated magnetized synthetic antiferromagnetic (SyAF)reference layer, which is mentioned as a preferred structure in thediscussion of FIG. 1 b. The SyAF layer of FIG. 2 would correspond tolayer (30) of FIG. 1 b. The SyAF includes a seed layer (31) suitable forthe growth of a layer of antiferromagnetic material (AFM). On the seedlayer there is an AFM layer (33) which will serve as a pinning layer. Onthe AFM pinning layer there is formed a tri-layer comprising a firstferromagnetic layer (35), an antiferromagnetically coupling layer (37)and a second ferromagnetic layer (39). The first and secondferromagnetic layers are magnetized in opposite directions (arrows (32)and (34)), the magnetization being rendered energetically favorable byexchange coupling across the coupling layer and being held in place bythe pinning layer. In this preferred embodiment, the two ferromagneticlayers are layers of CoFe or Co and are formed to thicknesses betweenapproximately 10 and 40 angstroms. The coupling layer (37) is a layer ofRu of thickness between approximately 7 and 8 angstroms, with 7.5angstroms being preferred or a layer of Rh of thickness betweenapproximately 4.5 and 5.5 angstroms with approximately 5 angstroms beingpreferred. The AFM layer (33) is a layer of MnPt or NiMn of thicknessbetween approximately 80 and 300 angstroms and the seed layer is a layerof NiFeCr, NiCr or NiFe of thickness between approximately 20 and 50angstroms. A SyAF reference layer such as this generates a very strongmagnetic pinning force on the sense layer.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than being limiting of the present invention. Revisionsand modifications may be made to methods, processes, materials,structures, and dimensions through which is formed a magnetic memorycell having a top keeper structure, while still providing a magneticmemory cell having a top keeper structure, formed in accord with thepresent invention as defined by the appended claims.

1. A method for forming a magnetic tunneling junction (MTJ) memory cell having a flux-concentrating keeper structure on an upper conductor comprising: providing a substrate; forming on the substrate a planarized linear lower conductor layer having a width W₁; forming on said lower conductor layer a patterned, MTJ stack, said stack being centrally aligned and co-linear with said lower conductor layer and said stack having width W₂ which is less than width W₁; forming an insulating layer contiguous with and laterally disposed to each side of said MTJ stack, an upper surface of said insulating and an upper surface of said MTJ stack forming a common plane; forming an upper linear conductor on said plane, said conductor contacting said MTJ structure, said conductor being transverse to said MTJ structure, said conductor having a keeper structure and said conductor being formed by a method comprising; forming a conducting layer on said plane; forming a first soft magnetic layer on said conducting layer; forming a capping layer on said soft magnetic layer; patterning said layers to form a linear structure with a planar horizontal upper surface and planar parallel vertical surfaces, the width of said structure, W₃, being substantially equal to W₂, said structure being transverse to said MTJ stack and an upper surface of said MTJ stack being thereby exposed laterally on either side of said structure; removing, using a first etching process aligned by said vertical surfaces, an upper portion of said MTJ stack laterally disposed to either side of said linear structure; forming a second, continuous soft magnetic layer on exposed surfaces of said MTJ stack and on horizontal and vertical surfaces of said linear structure, said layer having an outer surface including vertical surface portions laterally disposed to either side of said linear structure and substantially parallel to said vertical sides; removing, using a second, etching process aligned by said vertical outer surface portions of said second soft magnetic layer, all horizontal portions of said second soft magnetic layer, leaving, thereby, only vertical portions contacting vertical sides of said linear conductor and exposing, thereby, an upper horizontal surface of said linear structure and an upper surface of said MTJ structure.
 2. The method of claim 1 wherein the formation of said MTJ stack further comprises: forming a reference layer on said lower conductor layer; forming a tunneling barrier layer on said reference layer; forming a ferromagnetic sense layer on said tunneling barrier layer; forming a first capping layer on said sense layer patterning, using a photolithographic process, said layers to form an MTJ stack having parallel vertical planar sides, a horizontal upper surface and a width, W₂ defined by the distance between said vertical sides.
 3. The method of claim 2 wherein said sense layer is a layer of CoFe, CoFe/NiFe, NiFe, CoFeB, NiFeB or Co formed to a thickness between approximately 30 and 200 angstroms.
 4. The method of claim 2 wherein said tunneling barrier layer is a layer of Al₂O₃ or HfO formed to a thickness between approximately 5 and 30 angstroms.
 5. The method of claim 2 wherein said reference layer is a layer of CoFe, CoFe/NiFe, NiFe, CoFeB, NiFeB or Co formed to a thickness between approximately 30 and 200 angstroms.
 6. The method of claim 2 wherein said reference layer is a synthetic antiferromagnetic (SyAF) multi-layer formed by a method comprising: forming a seed layer on said lower conductor layer; forming an antiferromagnetic pinning layer on said seed layer; forming a first ferromagnetic layer on said pinning layer; forming an antiferromagnetically coupling layer on said first ferromagnetic layer; forming a second ferromagnetic layer on said coupling layer.
 7. The method of claim 6 wherein said first and second ferromagnetic layers are layers of CoFe or Co formed to thicknesses between approximately 10 and 40 angstroms.
 8. The method of claim 6 wherein said coupling layer is a layer of Ru formed to a thickness between approximately 7 and 8 angstroms or a layer of Rh formed to a thickness between approximately 4.5 and 5.5 angstroms.
 9. The method of claim 6 wherein said antiferromagnetic layer is a layer of MnPt or NiMn formed to a thickness between approximately 80 and 300 angstroms.
 10. The method of claim 6 wherein said seed layer is a layer of NiFe, NiFeCr, or NiCr, formed to a thickness between approximately 20 and 50 angstroms.
 11. The method of claim 2 wherein said first etching process is an ion-beam etch or a reactive ion etch (RIE) and wherein said removal of laterally disposed portions of said upper portion of the MTJ structure includes the complete removal of laterally disposed portions of said capping layer and a partial removal of laterally disposed portions of said sense layer formed beneath said capping layer, leaving laterally extending portions of said sense layer.
 12. The method of claim 2 wherein said second etching process is a vertically directed ion-beam etch and the upper surface of the MTJ structure exposed by said vertically directed ion-beam etch comprises those portions of the tunneling barrier layer laterally disposed to said vertical sides of said second soft magnetic layer.
 13. The method of claim 1 wherein said first soft magnetic layer includes an antiferromagnetic layer of IrMn formed to a thickness between approximately 30 and 100 angstroms, said antiferromagnetic layer contacting said conducting layer.
 14. An MTJ memory device including an upper conductor with a keeper structure comprising: a substrate; an insulating layer formed on said substrate, a lower conductor layer being formed in said insulating layer and the upper surfaces of said insulating layer and said conductor being co-planar; an MTJ stack formed, co-linearly and centrally, on said lower conductor, said stack including a sense layer; an insulating layer formed, laterally disposed to both sides of said stack, the upper surfaces of said layer and the upper surface of said stack forming a common plane; a horizontally layered upper conductor formed orthogonally to and contacting said MTJ stack on said plane, said upper conductor having planar vertical sides and the upper layer of said conductor being a layer of soft magnetic material; a keeper structure formed on said upper conductor, said structure comprising said upper soft magnetic layer contacted on laterally opposite edges by an upper portion of vertical soft magnetic side pieces formed on said planar vertical sides, a lower portion of said vertical side pieces contacting sides of a sense layer and terminating on lateral extensions of said sense layer. 